1. Field of the Invention
The present relates generally to methods and apparatus for improving the efficiency of the drilling process and also conducting surveys of drilled wells, particularly directional wells having wellbores that transition from vertical to inclined to horizontal orientation for intersecting a desired subsurface anomoly or target. More specifically, the present invention is directed to a method for increasing the efficiency of the drilling operation by minimizing the number of high accuracy stationary measurements by integrating or augmenting that measurement sequence with an additional sequence of lower accuracy measurements that can be made without having to suspend the drilling operation. A particular embodiment of this invention is a method for developing a well survey wherein continuous inclination data, typically achieved by a measuring while drilling (xe2x80x9cMWDxe2x80x9d) tool, is integrated with or used to augment the minimum radius of curvature approximation of the trajectory shape between conventional survey points taken along a wellbore trajectory to define individual arcs and tangents in the trajectory and thereby improve the accuracy of the wellbore""s position.
2. Description of the Related Art
In drilling a directional well, it is common to use a bottom hole drilling assembly (BHA) that is attached to a drill collar as part of the drill string. This BHA typically includes, in descending order, a drilling motor assembly, a drive shaft system including a bit box and a drill bit. In addition to the motor, the drilling motor assembly may include a bent housing assembly which has a small bend angle in the lower portion of the BHA. This bend angle causes the borehole being drilled to curve and gradually establish a new borehole inclination and/or azimuth. During the drilling of a borehole, if the drill string is not rotated, but merely slides downward as the drill bit is being driven only by the motor, the inclination and/or the azimuth of the borehole will gradually change, in other words, curve, due to the bend angle, thus forming a curved wellbore section. Depending upon the tool face angle, that is, the angle at which the drill bit is pointing relative to the high side of the borehole, the borehole can be made to curve at a given azimuth and inclination. If however, the rotation of the drill string is superimposed over that of the output shaft of the motor, the bend point will simply travel around the axis of the borehole so that the bit normally will drill straight ahead at whatever inclination and azimuth have been previously established, thus forming a straight wellbore section. The type of drilling motor that is provided with a bent housing is normally referred to as a steerable drilling system. When drilling with a steerable drilling system of this nature, various combinations of sliding and rotating drilling procedures can be used to control the borehole trajectory in a manner such that eventually the drilling of a borehole will proceed to a targeted formation. Stabilizers, a bent sub, and a kick pad also can be used to control the angle build rate in sliding drilling, or to ensure the stability of the wellbore trajectory in the rotating mode. Thus, when the drill string is not being rotated and the drill bit is being rotated by the drilling motor in a steerable or directional drilling system, the wellbore segment being drilled will be of curved geometry. Likewise, when the drill string is being rotated and the drilling motor is also being operated, the resulting wellbore section being drilled will be substantially straight.
During well drilling, to confirm the spatial position of the wellbore being drilled as it progresses through the formation, it is necessary to conduct periodic well surveys, either using a well survey instrument or using the various sensors of a measuring while drilling (MWD) tool that is incorporated within the well drilling string. These periodic well surveys establish survey stations at selected intervals along the length of the wellbore. Typically, between survey stations the wellbore will be defined by a number of straight wellbore sections or tangents which result from drilling with the drill motor and simultaneously rotating the drill string and a number of curved wellbore sections or arcs which result from drilling only with the drill motor without rotating the drill string, so that the drill string merely slides along the curved wellbore section being drilled. While the survey stations are typically located at substantially equally spaced locations along the wellbore, typically determined by the lengths of the drill pipe sections or the length of the stands of drill pipe, the lengths of the arcs and tangents will vary according to bent motor orientation during drilling. It is typical to compute the trajectory of a wellbore by using a minimum radius of curvature algorithm which assumes that the geometry of a wellbore between survey stations lies along a smoothly curved arc. Well surveys being calculated from the data and the survey points can have significant error because the actual geometry of the drilled wellbore in most cases will not lie along a curve of fixed curvature but rather will consist of a plurality of arcs and tangents arranged end to end and having a bend angle at the juncture of the arcs and tangents. Thus, the spatial position of the wellbore at any given depth can be sufficiently in error that an intended target can be missed.
The same general wellbore geometry is established, as indicated above, when rotary steerable drilling systems are employed for well drilling activities. A rotary steerable drilling tool typically includes a drill collar that is rotated by a drill string and supports a bit shaft to which a drill bit is fixed. The bit shaft is angularly adjusted relative to the drill collar about a pivot mount within the drill collar. As rotary drilling progresses, the angular position of the bit shaft and thus the drill bit is adjusted, in other words, steered by steering control signals communicated from the surface or by on-board sensor responsive steering signals to define straight borehole sections or curved borehole sections having periodically controlled inclination and azimuth to progress the wellbore toward an intended subsurface target. The result of this steerable drilling with a rotary steerable drilling tool is that the wellbore being drilled will be defined by a series of arcs and tangents in the same manner as discussed above. For accurately determining the spatial position of the wellbore at any desired location between the survey points that are achieved at intervals when drilling is discontinued, the trajectory shape of the wellbore between the survey points is desired.
Historically, well surveys were, and in many cases are conducted by running into a wellbore a well survey sonde having a housing that is selectively positioned by cable equipment. The cable equipment typically incorporates electrical conductors for conducting various position signals from on-board sensors of the survey tool to surface equipment for receiving and processing the signals. The survey instrument will typically incorporate one or more inclinometers and an orthogonal triad of accelerometers for measuring the angle of the local vertical with respect to the sonde. Since the sonde handling cable does not control rotational positioning of the sonde, it is necessary that the surveying instrument have the capability for measuring probe orientation to provide a reference for the inclinometer measurements and thus enable measurement of the azimuth of the borehole at the survey point or station. Sonde orientation may be measured with gyroscopes or magnetometers which may be utilized independently or in conjunction with other position sensing systems. When a borehole is surveyed using a sonde or survey instrument of this nature the result is typically a series of survey points or stations at fairly widely spaced intervals along the wellbore. The survey points, which are accurate from the standpoint of inclination and azimuth, are recorded and are then processed by the minimum radius of curvature algorithm, or any other similar algorithm to approximate the geometry, inclination and azimuth of the wellbore between the survey points. Though these widely spaced survey points can be utilized to fairly closely approximate a curved wellbore substantially incorporating the survey points, the true geometry of the wellbore cannot be accurately determined in this manner. However, for accurate determination of spatial position of the wellbore at any desired location along its length, it is highly desirable to have the ability of accurately measuring the arcs and tangents and correlating such measurements with the survey station measurements.
The current method of computing a directional wellbore""s spatial position is to integrate from the surface or from a known point along a well path which is defined by a series of survey points. These survey points give the inclination and azimuth of the wellpath at specific depths, and are indexed in measured depth. The minimum radius of curvature algorithm is used to interpolate between the survey points. However if the directional well is drilled with a bent motor, the real trajectory of the wellbore will consist of a series of arcs, curved wellbore sections, and tangents, straight wellbore sections, as the drilling motor is slid or rotated. If the along hole length of the slide or tangent sections of the wellbore is less than the survey interval, then the minimum radius of curvature algorithm models the trajectory as one single continuous are with a constant radius of curvature, overestimating the true vertical depth of the well. It is desirable therefore to provide a well survey system which takes into account accurate well survey signals that are acquired at the widely spaced survey stations and which also take into account substantially continuous inclination measurement data that is acquired during drilling. A continuous inclination measurement can be used to define the individual arcs and tangents in the borehole trajectory between survey stations, thereby improving the accuracy of the integration, and therefore the accuracy of the wellbore""s spatial position.
In addition to measuring the well-bore trajectory, it is usually convenient and often necessary to make additional measurements from within the well-bore while it is being drilled. As with the well-bore surveys, these additional measurements can generally be made to a higher accuracy or resolution when the tool is stationary. For example, formation pressure measurements can be made from sensors positioned on the drillpipe if such sensors can be extended into the formation as probes or by inflating packers to isolate such sensors from the hydrostatic pressure above and below the tool. This operation requires stopping drilling. Approximate measurements or inferences of pressure, however, can be made while drilling. For example, within a particular sedimentary basin it is possible to derive empirical relationships between formation resistivity, porosity and formation pressure. Measurements of resistivity and porosity can be made without suspending the drilling operation, but inferences of formation pressure from such measurements are inherently less accurate than a direct measurement and may commonly also suffer from a bias offset or gain. It is desirable therefore to provide a combined measurement system which takes into account the well measurements acquired at widely spaced intervals and which also takes into account the well measurements that are obtained from a substantially continuous measurement made while drilling.
It is a principal feature of the present invention to provide a novel method for increasing the efficiency of the drilling process by providing a method which accurately measures properties of the formation, the wellbore trajectory or the drilling processes itself while at the same time minimizing any requirement to suspend that drilling process.
It is also a principal feature of the present invention to provide a novel method for accurately measuring the spatial position of a wellbore at any position or depth along the wellbore to more efficiently provide for steering of a wellbore to an intended subsurface target;
It is another feature of the present invention to provide a novel method in which complete and highly accurate well surveys can be taken less frequently by adding a larger number of substantially continuous measurements albeit of lower accuracy.
It is also a feature of the present invention to provide a novel method, providing a process for combining highly accurate survey data with lower quality survey data in such a way that the accuracy of the overall borehole trajectory is improved.
It is another feature of the present invention to provide a novel method for measuring curved and tangent segments of a wellbore by continuous inclination measurements during drilling and by augmenting the minimum radius of curvature calculation between spaced survey points along the wellbore with the continuous inclination measurements to thus provide for significantly enhanced accuracy of the calculated spatial position of the wellbore at any location along the length thereof.
It is also a feature of the present invention to provide a novel method for wellbore surveying wherein substantially continuous inclination measurements of the wellbore are acquired by a MWD tool during drilling, periodic survey points are established along the wellbore with the MWD tool or other measurement tool static within the wellbore and then integrating the survey point measurements with the substantially continuous inclination measurements to achieve highly accurate measurement of the spatial position of the wellbore.
It is an even further feature of the present invention to provide a novel method for wellbore surveying wherein a substantially continuous inclination log is established between survey points and a gain and offset are applied to each section, forcing a match between the continuous inclination measurements and the station inclination measurements at the endpoints of each section and confirming agreement at the endpoints.
Briefly, the invention provides a method that includes the steps of taking discrete measurements of a well or formation parameter having a first accuracy when drilling is substantially suspended, and taking substantially continuous measurements of the well or formation parameter having a second accuracy during a drilling operation. The second accuracy has a reduced accuracy compared to the first accuracy. The measurements having the first and second accuracies are then combined, whereby the well may be efficiently drilled.
In a particular embodiment, the invention further contemplates the step of applying the combined measurements to other sets of discrete measurements taken while drilling is substantially suspended.
The invention further provides a method of drilling of a well which includes the steps of taking discrete measurements of a well parameter with a first instrument when drilling is substantially suspended, taking substantially continuous measurements of the well parameter with a second instrument during drilling, the second instrument having reduced accuracy compared to the first instrument, and combining the measurements from the first and second instruments to maximize the accuracy of the measurements taken with the first instrument. In this manner, the well may be efficiently drilled and the utility of both sets of measurements is maximized.
For example, efficient drilling may take advantage of long stands of drillpipe is often impeded by the need to take a full survey every 30 feet of drillpipe. The invention described herein details a process by which complete and highly accurate measurements can be taken less frequently by adding a larger number of substantially continuous measurements albeit of lower accuracy.
In a particular embodiment, the method can be used with continuous inclination and/or azimuthal data determined between survey stations, whereby the survey station measurements are augmented with the continuous inclination measurements to improve the accuracy of the calculated results of a minimum radius of curvature computation or any similar algorithm. The particular embodiment is therefore a process to combine highly accurate survey data with lower quality survey data in such manner that the accuracy of the overall borehole trajectory is improved.
The inclination and azimuth at the survey stations is acquired with the MWD tool of the drilling system stationary so that drilling noise will not be present during acquisition of the survey points. Continuous survey measurements are acquired during the drilling process, therefore drilling noise is present. Survey points established with the drilling system stationary are more accurate but infrequently sampled because drilling is stopped to facilitate the survey. Continuous surveying is less accurate, because of the presence of drilling noise, but is more frequently sampled because it can be done while drilling is in progress. The objective of the particular embodiment is to augment the minimum radius of curvature approximation of the trajectory shape between the survey stations with whatever reliable information can be extracted from the continuous inclination measurements. Additionally, from the standpoint of signal processing, the continuous inclination measurements can be electronically filtered to minimize the influence of drilling noise and thereby enhance the vitality of the resulting measurements.